1. Field of the Invention
The present invention relates to a light shielding blade material for use in an optical apparatus that requires a blade having a good light shielding property, a light weight and a high rigidity for use as a shutter blade in a focal plane shutter or a lens shutter, or for use as a diaphragm blade of a camera.
2. Description of the Related Art
A blade such as a shutter blade and a diaphragm blade for use in an optical apparatus such as a camera must be light in weight and must be high in rigidity, because the blade runs and stops in an extremely short time at a fast speed. Also, the blade must intrinsically have a good light shielding property, because the blade covers photosensitive materials such as a photographic film at the front side thereof to block an incident light from impinging onto the photosensitive materials. Besides, the blade must have a good lubricity for a smooth operation, because a plurality of blades are generally arranged to overlap with each other and to operate in the overlapping state. Further, the blade must have a low surface reflectivity and a good planarity to prevent the incident light from leaking through a clearance between the blades. The planarity is also important for preventing a breakage due to a collision between the blades during operation.
Disclosures of prior art references will be described briefly. Japanese Examined (Granted) Utility Model Publication No. 53-12192 discloses a technique to obtain a practical light shielding property by laminating a metal film on a shutter film of an optical apparatus by means of vapor deposition or sputtering in case that the light shielding property of the shutter film is insufficient. Japanese Patent Laid-Open Publication (Tokkai) No.49-84232 discloses that a thin thermosetting resin sheet reinforced with carbon fibers is applied to the shutter blade and that a black pigment such as a carbon black is added to the matrix resin to obtain the practical light shielding property. Japanese Patent Laid-Open Publication No. 51-14969 discloses a laminate structure of carbon fiber reinforced composite resin sheets having less warping. Japanese Patent Laid-Open Publication No. 53-101080 discloses a carbon fiber reinforced composite resin laminate, which is light in weight and which has a high strength, a high elasticity, a high impact resistance and a vibration damping property, and suggests that the carbon fiber reinforced composite resin laminate has suitable properties as a light shielding blade for use in an optical apparatus such as a camera that repeats a high speed operation of run and stop. Japanese Patent Laid-Open Publication No. 59-61827 discloses a composite member used for the light shielding blade comprising a core layer with tiny cavities and surface layers. At least, one of the core and the surface layers is reinforced with continuous carbon fibers. Further, the Publication teaches that a black die and a carbon particulate are added to a carbon fiber reinforced plastic (CFRP) matrix resin in case that the light shielding property is insufficient. Japanese Patent Laid-Open Publication No. 64-85749 discloses a blade material comprising a biaxially stretching plastic film substrate having different thermal contraction factors between a primary stretching direction and a secondary stretching direction, and a carbon fiber reinforced resin sheet is applied to both surfaces of the substrate. According to the Publication, the lamination structure is constituted so that a direction with a great thermal contraction factor (i.e., the primary stretching direction) of the substrate is substantially orthogonal to an alignment direction of the carbon fibers contained in the carbon fiber reinforced resin. Japanese Patent Laid-Open Publication No. 57-60315 discloses a shatter blade used in a camera and made of a plurality of crystal polymer compound film layers such as polyester film layers and at least one metal layer interposed therebetween as a light shielding means. The Publication teaches that one or more of coating films containing, for example, a black paint may be interposed between the plastic films as a light shielding means. Also, the Publication discloses that a black pigment or a black die may be added to at least one of the plastic films. Japanese Patent Laid-Open Publication No.57-118226 discloses a plastic blade used for an optical apparatus and made by coating a biaxially stretching polyester film with a thermosetting mat paint and by adhering an anti-electrostatic agent. The biaxially stretching polyester film has a thickness of approximately 100 xcexcm or less, and has an optical density of approximately 10 when the film thickness has approximately 70 xcexcm. Japanese Patent Laid-Open Publication No. 9-274218 discloses that a light shielding film is made by using a film mainly containing a thermosetting resin as a substrate film, and by applying a thermosetting resin layer containing a carbon black, a lubricant and a delustering agent to both surfaces of the film. As the substrate film, a biaxially stretching polyethylene terephthalate (PET) film containing a carbon black is used to obtain the good light shielding property.
In general, the light shielding blade for the optical apparatus requires a good light shielding property, a light weight, a high rigidity, and other properties such as mechanical properties, thermal properties, chemical properties and physical properties, planarity, workability (i.e., serviceability), and a long lifetime (i.e., corrosion resistance). Upon application of a plastic film material or a carbon fiber reinforced composite resin material to the light shielding blade for the optical apparatus, the most important property is practically the light shielding property. A light shielding plastic blade containing a black pigment such as carbon black with a good light shielding property has been practically used. For example, the carbon black is added to a PET film to provide a black PET film material having a film thickness of 50 to 125 xcexcm with the excellent light shielding property and no pinholes, and having an optical density of 8 or more. Further, a light shielding blade for an optical apparatus in which a black lubricant coating is formed on the black PET film material is practically used, as disclosed, for example, in Japanese Patent Laid-Open Publication No. 9-274218 as described above. However, such a black PET film obtained by adding a high concentration of the carbon black to adjust the optical density of 8 or more is extremely special and expensive. A film material having the optical density of 8 or less available in a relative low price cannot be used as a blade for a purpose of shielding an incident light. In this case, it is required to form a metal film by vapor deposition, sputtering or wet plating and further to form a black lubricant coating on the metal film, as is disclosed in Japanese Examined (granted) Utility Model Publication No. 53-12192 described above. Traditionally, a paint coating cannot shield the light completely, and a metal film should be used to obtain the perfect shield of the light.
Upon applying a plastic film material to a light shielding blade for an optical apparatus, the planarity is another important feature as well as the light shielding property described above. One significant problem is a low yield caused by a poor planarity in applying a carbon fiber composite resin laminate (CFRP) to the light shielding blade for the optical apparatus. In addition, the CFRP has an insufficient light shielding property due to kink of the fibers. To improve the light shielding property, the carbon black may be kneaded into the matrix resin. However, a prepreg sheet in which the carbon black is kneaded in a specially high grade and a laminate obtained from such a prepreg sheet are quite expensive.
To solve the above-mentioned conventional problems, the following measures are taken. Namely, a plastic light shielding blade material for an optical apparatus according to the present invention comprises a plastic film substrate, light shield coatings disposed on both surfaces of the plastic film substrate, and black lubricant coatings disposed on the light shield coatings through respective reinforcement members. Namely, the reinforcement members are interposed between the light shield coatings and the black lubricant coatings to obtain a laminate structure. The reinforcement members are composed of thermosetting resin prepreg sheets laminated and hardened during fabrication of the light shielding blade material. The reinforcement member is reinforced with continuous fibers drawn in an alignment direction. By such a manner, an upper layer of the lubricant coating and a lower layer of the shield coating are separated from each other by an intermediate layer of the reinforcement member. Preferably, the drawn continuous fibers contained in the reinforcement member disposed on the substrate are aligned in parallel to those contained in the other reinforcement member. The substrate comprises a biaxially stretching plastic film. The primary stretching direction can be set freely relative to the alignment direction of the continuous fibers. Preferably, the light shield coating is composed of a paint resin containing carbon black, and the reinforcement member is composed of a thermosetting resin containing no carbon black. Preferably, the reinforcement member is reinforced with carbon fibers. Otherwise, the reinforcement member may be reinforced with polyparaphenylene benzobisoxazole fibers. Preferably, the substrate comprises a transparent plastic film having an optical density of zero or a non-transparent plastic film kneaded with carbon black and having an optical density of 8 or less. As the substrate, a polyethylene terephthalate film, a polyethylene naphthalate film or an aramid film is used. Preferably, the light shield coating contains 20 to 45% by weight of carbon black. A sum of the optical density of one light shield coating and the optical density of the substrate is 6 or more. A total optical density of the inventive laminate structure including both of the light shield coatings is 12 or more. In case that the substrate has a less adhesion to the light shield coatings, the substrate may be treated to improve adhesiveness such as a primer treatment, a plasma treatment, a chemical conversion treatment and a blast treatment.
According to the present invention, the biaxially stretching plastic film having a thickness of, for example, 12 to 125 xcexcm is used as the substrate of the light shielding blade material for the optical apparatus. To assure the light shielding property required for the optical apparatus, the resin coating i.e., the light shield coating, is comprised of epoxy, diallylphthalate, acrylic, urethane, vinyl chloride or vinyl acetate, containing 20 to 45% by weight of carbon black as a dried film. The light shield coating is formed on both surfaces of the biaxially stretching plastic film in a thickness of 2 to 8 xcexcm by dipping or coating using a roll coater. A total thickness of the resin coatings is in the order of 4 to 16 xcexcm. The substrate, the light shield coatings and the lubricant coatings constitute the laminate having a total optical density in the order of 12 or more. Even after peeling one side of the light shield coatings and one side of the lubricant coatings, the total optical density maintains 6 or more. The plastic light shielding blade material for the optical apparatus that satisfies both the light weight and the optical shielding property can be thus obtained. The light shield coatings are located under the black lubricant coatings. Accordingly, a loss of the light shielding property is never created due to abrasion during a shutter operation. Normally, the plastic film substrate does not satisfy a required strength singly, hence carbon fiber composite thin sheets are, for example, used as the reinforcement members. The intermediate reinforcement member is interposed between the upper lubricant coating and the lower shield coating to allocate different functions to these separate layers. Namely, the shield coating performs the optical blocking function while physically protected by the reinforcement member, and the lubricant coating performs the lubricating function to reduce a surface friction during a shutter operation. There is no need to knead carbon black into a matrix resin of a prepreg sheet of the carbon fiber composite reinforcement member. With the light shield coatings described above alone, the complete light shielding property can be obtained. Thus, the light shield coatings formed on both surfaces of the film substrate can ensure the light shielding property required for the optical apparatus such as a camera. Consequently, there is no need to add carbon black and the like to the substrate to achieve the required light shielding property. Furthermore, there is no need to use a metal film to ensure the light shielding property, thereby realizing a reduction of weight.